The present invention relates generally to aircraft transparencies, or windshields and canopies, and more particularly to frameless transparencies.
Transparent cockpit enclosures for fighter and trainer type high performance aircraft typically comprise a plurality of aircraft transparencies. The transparencies comprise curved transparent panels surrounded by frames providing both structural support and means for attaching the transparencies to aircraft sill structures. The frames are usually fabricated of metal or a matrix of high strength fibers, and are attached to the transparent panels by bonding agents or fasteners.
Large discontinuities in material properties exist at the panel and frame boundaries, creating design complications for structurally withstanding the dynamic loading associated with bird impact. Rows of bolt holes near transparent panel edges in most transparancies weaken the panels in that critical structural area. The panel and frame design requices pressure seals at both the panel and frame interface and at the frame and aircraft sill structure interface. Stresses occur in the transparent panels due to frame installation and differential thermal expansion. These stresses lead to structural deficiencies, optical distortions and limited durability. The frames themselves are generally made of metals such as magnesium, having high static strength, but brittle in response to dynamic loads such as bird strikes.
U.S. Pat. No. 1,004,388 to Stefanik discloses a common approach in use today for the problems accompanying panel and frame design. Stefanik teaches overlapping the edges of a transparent panel with uncured laminated fiberglass strips, then curing the strips under heat and pressure to form rigid connecting members firmly secured to the transparent panel. These reinforced plastic edge reinforcements are then drilled for attachment to a metal frame which attaches to the aircraft sill structure.
Prior art attempted solutions toward producing a "frameless" transparency include U.S. Pat. No. 2,511,168 to Martin. et al., which teaches cementing a mounting strip into a slot formed inside the transparent panel edges. The mounting strip is formed of the same plastic as the transparent panel, but reinforced by impregnated layers of metal wire or screen. The reinforced strip is fastened into slots or other fastening locations on the aircraft sill structures. Martin adds sufficient additional mechanical structure to prevent the structure from being truly frameless and does not allow opening and closing for pilot ingress and egress without the addition of a separate frame.
U.S. Pat. No. 2,637,076 to Bolte describes an outer frame of intersecting ribs and spars of transparent plastic cemented and reinforced with transparent tape. Bolte does not describe a sill attachment structure.
U.S. Pat. No. 2,258,721 to H. Wagner. et al. teaches forming the edges of the transparent panels into pear-shaped beads to fit within correspondingly shaped recesses in the aircraft sill structures. This design creates a "step" between the outside of the sill structure and the transparent panel, preventing a smooth aerodynamic transition from the outside of the sill structure to the transparent panel. Wagner does not allow opening and closing for pilot ingress and egress without the addition of a separate frame.
U.S. Pat. No. 4,081,581 to Littell discloses a laminated windshield design wherein the inside laminations extend beyond the windshield edges to allow placement of bolt holes for attachment. Littell teaches that, while the bolt holes are primarily for use in attaching the windshield to test apparatus, they may possibly be used for direct attachment of windshields to aircraft. However, as with the other prior art structures thus far described, the use of this design in canopies, which generally have to structurally support hinges and latches for opening to allow pilot ingress and egress, still will require edge reinforcements and a frame.
Acrylic plastics have been the most often used material for transparent panels in the past. Acrylics offer light weight and good formability, but are typically too brittle to resist bird impact or to be used without a frame. The recent introduction of tougher polycarbonate and other plastics for their bird impact resistance offers the possibility of using that toughness to build frameless transparencies. The mere substitution of modern plastics for acrylic, however, is not sufficient, as indicated by the recognition in Littell, describing a polycarbonate laminate, of the continued need for edge reinforcements and frames.
Polycarbonate transparencies in use today are made from layers of thin polycarbonate sheets laminated with intervening layers of elastomeric resin. Curved transparencies are generally made by bending and forming extruded flat sheets of plastic under heat and pressure. Applied to thicker polycarbonate panels, this process results in unpredictable physical properties. Laminating thinner sheets solves that problem, but limits the design options available, thus preventing successful utilization of polycarbonate toughness for aspects of transparency design other than for bird strike protection.
Elimination of transparency frames will save cost, complexity and weight. It will minimize the required inventory of parts and fasteners. It will enhance flight safety by improving bird impact resistance and will lower manufacturing costs. It will reduce transparency change-out time and cost. It will reduce corrosion problems related to transparency frames and fasteners. It will minimize structural discontinuities at the transparency and frame interface, providing a wealth of as yet unanticipated advantages.
It is, therefore, a principal object of the present invention to provide an improved frameless transparency.
Another object of the present invention is to provide an improved monolithic transparency.
Yet another object of the present invention is to provide a frameless transparency that provides for canopy opening and closing relative to the aircraft sill structure.
A further object of the present invention is to provide a transparency that reduces the required number of fasteners and parts.
These and other objects of the present invention will become apparent as the detailed description of certain representative embodiments proceeds.